During the operation of a machine, it may be desired to change the machine's direction of propulsion (i.e., forward or backward). When such a direction change is requested, a machine control system initially controls a power train to retard motion in the current direction. Typically, the retarding event is initiated by adjusting the transmission (e.g. downshifting) to drive power into the engine, thereby using parasitic losses to slow the machine down. Driving the power into the engine typically causes the engine speed to increase. If the engine is supplied with fuel during the retarding phase, the engine will generate power that can conflict with the power being dissipated. This may reduce the retarding capability of the engine. Therefore, during retarding events, the supply of fuel to the engine is reduced or cut off.
The retarding phase of the propulsion direction change event ends when the machine starts traveling in the opposite direction and the acceleration phase begins. During the acceleration phase, the transmission no longer directs power into the engine, and the parasitic losses cause the engine slow down. In addition, the control system places an acceleration load on the engine, which causes the engine to transfer to the transmission, any power absorbed during the retarding phase. This further contributes to the decrease in engine speed. To counter the decrease in engine speed and generate enough torque to meet the acceleration demand, the control system resumes or increases the supply of fuel to the engine. However, there is a delay between the resumption or increase of fuel and the generation of an adequate amount of torque to meet the acceleration demand. During this delay, the engine speed may continue to decline and may even fall below a minimum desired speed. While operating below the minimum desired speed, the engine's performance may be unsatisfactory.
One method that has been employed to minimize the amount of time an engine operates at speeds below the minimum desired speed during a propulsion direction change event is disclosed in U.S. Pat. No. 4,769,774 (the '774 patent) issued to Narita et al. on Sep. 6, 1988. During a propulsion direction change event, the system disclosed in the '774 patent determines when the engine speed is accelerating and terminates fueling. As the propulsion direction change event is performed, the system continually monitors the vehicular speed and the engine speed. When the vehicle's ground speed approaches zero or the engine speed approaches a speed that may cause the engine to stall, the system resumes fueling and operates the power train. This allows the control system to resume fueling before the vehicle is propelled in the opposite direction.
Although the system disclosed in the '774 patent resumes fueling earlier in the propulsion change event, the performance of the engine may still be unsatisfactory. In particular, while in the propulsion direction change mode, the power train continues to operate according to a map used when the vehicle is not performing a propulsion direction change event. The focus of operation during non propulsion direction change events may be different from the focus of operation during propulsion direction change events. For example, during a non propulsion direction change event, the focus may be on fuel economy, and the vehicle may operate according to a fuel economy map. However, engine power is sacrificed when operating according to a fuel economy map. Such engine power is needed to accelerate the vehicle in the acceleration phase of the propulsion direction change event. By continuing to operate according a previously selected map, the power train may not be able to meet the demands imposed on it, and its performance may be unsatisfactory.
The disclosed system is directed to overcoming one or more of the problems set forth above.